For decades, a serious and longstanding concern of providers of artistic or expressive content has been “piracy,” or access to such content by people unauthorized or unlicensed to do so. Such piracy may result in significant revenue losses for the content provider. Simultaneously, support for the principle of fair use may require allowing consumers to access content in ways not foreseen by the provider. While the now-commonplace digitization of many types of content, such as music, film, video and literary work, has resulted in efficient, effective distribution of the content, digitization has also made piracy of the content easier and more widespread, as the content may be duplicated quickly without reduction in quality any number of times and without regard to whether the resulting copy is of a second or later generation.
In an effort to prevent piracy of digitized content, providers have often employed various technological measures as part of a Digital Rights Management (DRM) scheme, which typically outlines the rights and restrictions associated with the use of the content, and provides the technological measures and related requirements for enforcing the license. One example of a DRM scheme is the Content Scrambling System (CSS) often employed to prevent viewing of Digital Video Discs (DVDs) on unlicensed or unauthorized DVD players. Many other DRM schemes associated with various forms of digital content have also been utilized, with varying success in allowing authorized users to access the protected content while preventing piracy.
In some DRM schemes, a content provider may set a time limit on the accessibility of the content by the user. For example, a user may pay to rent a particular work, such as an audio or video presentation. In return, the provider may allow access to the content for a limited period of time, beginning at some predetermined date and time, or starting with an initial access of the content by the user. To enforce these license restrictions, the provider may require that the device facilitating the user access to the content include a system clock capable of enforcing the provider's time restrictions. Further, to hinder the user from circumventing the license restrictions, the provider may call for the particular clock system employed to be secured against potential user tampering.
In one particular example of a time-based restriction, a user may only have until Friday at noon to access the rented content. If the user wishes to access the content on Saturday, he may attempt to turn back the current time value of the system clock to indicate Friday before noon so that the content may still be accessed. A secure system clock could be used to prevent such a modification.
Unfortunately, no matter how much care has been taken to make DRM technological measures foolproof, many anti-piracy schemes, including those employing the secure clocks mentioned above, may be defeated by motivated and technically astute individuals. In other words, despite attempts to prevent the user from accessing a secure clock, the user may be able to circumvent the clock security measures to modify the system date and time, thus allowing the user to access the content in violation of the terms set forth in the content license. In other cases, legitimate attempts of the user to access a system clock, such as to realign the clock with the current time to correct for inherent frequency “drift,” may unnecessarily be thwarted in order to maintain the security of the clock.